Abstract The levels and excitation energies of a weakly interacting AsGa - Asi defect pair have been calculated using the model energy functional introduced by Baraff and Schluter. The calculation is performed in a manner which allows the Jahn-Teller relaxation proposed by Lannoo to distort the Asi away from the symmetry site. We evaluate the parameters of the model using experimental information where possible and using the results of self consistent Greens function calculations where no experimental information is available. The isolated Asi turns out to have no equilibrium state capable of giving rise to an ESR spectrum. It has a shallow donor level plus a two-electron level (As+i , As 3+i ) in the lower half gap. This level structure persists in the defect pair, which now contains, in addition to the excitations associated with the individual defects, a charge transfer excitation in which (As 0Ga - As+i) → (As- Ga - As2+i ). The species As- Ga, which is unstable in isolation, becomes longer lived as a component of the defect pair. The calculations support the proposal of von Bardeleben et al. that the ground state of the metastable defect EL2 is, in fact, this defect pair.